Many times when you break up with your ex, they ask you to enter the gray area of friendship and be good friends.
But according to the saying, "A friendship can become love, but a love cannot become friendship," and he's not wrong.
Staying friends with your ex keeps you imprisoned in a past that crushes you and doesn't let you move forward with your life.
On the one hand, it's your ex's fault for asking you for something like this - he obviously wants you as his date and to have a good time until he finds something else, or maybe he didn't really feel anything for you or maybe he got over it and really wants you as a friend but it's a bit of a cliché - but on the other hand, it's also your fault for tolerating such a situation.
Being friends with him and having him in your life afterwards will create problems for you in your next relationship as you will still be stuck with your ex and won't be able to commit to your current one.
Additionally, you can't stay friends with your ex from the first day of your breakup, because it takes time for your wounds to heal, but also for you to decide whether to enter the friend zone.
Even if your ex comes and goes in your life, you are the one who can close the door on him once and for all because, as the saying goes,
"There are orange trees elsewhere that produce oranges."